When controlling the operation of a walking assist device to assist a human being in making a walking motion, for example, it is necessary to grasp joint moments actually acting on the joints of the legs of the human being. The grasp of the joint moments enables an appropriate determination of a target support force of the walking assist device. Also in a two-legged walking robot, there is sometimes a need to grasp the joint moments actually acting on the joints of the legs appropriately to control its operation.
Accordingly, the applicant of the present application has earlier suggested a method of estimating joint moments of the legs of a two-legged walking mobile body such as a human being in Japanese Patent Laid-Open No. 2003-89083 (hereinafter, referred to as the patent document 1) and the like, for example. This method includes measuring displacements (rotation angles) of respective joints of the legs of the two-legged walking mobile body or an acceleration and an angular velocity of a predetermined region by using a required sensor and estimating a floor reaction force vector acting on each of the legs and a position of the point of application of the floor reaction force vector by using the measured data or a rigid link model of the two-legged walking mobile body. In this regard, the rigid link model is a model in which the structure of the two-legged walking mobile body is represented as a link body composed of a plurality of rigid elements coupled together by a plurality of joint elements. The rigid link model is used to estimate an total center-of-gravity location of the two-legged walking mobile body and the positions and postures of rigid equivalent parts (a thigh, a crus, a waist and the like) and joints (a knee joint, a hip joint, and the like) of the two-legged walking mobile body corresponding to the rigid elements and the joint elements, respectively, and is also used as a basis of a model for describing kinetic behaviors of the two-legged walking mobile body. For each of the rigid elements of the rigid link model, the weight, the length, and the center of gravity location (the position on each rigid element) are preset concomitantly.
Moreover, in the method disclosed in the patent document 1, a joint moment of a knee joint or a hip joint of each leg is estimated by arithmetic processing based on an inverse dynamics model by using an estimated floor reaction force vector, a position of the point of application of the floor reaction force vector, and the rigid link model. Generally speaking, the inverse dynamics model is a dynamic model for use in estimating a reaction force, which is an internal force of an object, or a moment, supposing that an external force acting on the object and position information have already been known (with the external force and the position information as input parameters), and represents a relation between the motion of the object (a time-series pattern of a position) and a force or a moment acting on the object. In the method of the patent document 1, the inverse dynamics model is constructed based on an equation of motion related to the motion (translational motion and rotary motion) of each rigid element of the rigid link model and the joint moments of each leg are estimated in order from a joint moment on the side of a joint closer to the point of application of the floor reaction force vector.
When estimating the joint moments as in the patent document 1, the floor reaction force vector, the point of application of the floor reaction force vector, and the inverse dynamics model need be expressed in a single coordinate system. Then, in the patent document 1, an absolute coordinate system fixed to a floor has been used as the coordinate system.
If the absolute coordinate system is used in this manner, there is a need to grasp a tilt angle in the absolute coordinate system such as, for example, a tilt angle relative to the vertical direction, of a predetermined region of the two-legged walking mobile body (more specifically, a rigid equivalent part corresponding to a certain rigid element of the rigid link model) and at the same time to transform the position and the posture of each region of the two-legged walking mobile body (a rigid equivalent part corresponding to the rigid element of the rigid link model) grasped as the relative position and posture to a certain reference region of the two-legged walking mobile body to values in the absolute coordinate system. Moreover, when calculating a floor reaction force vector or a joint moment, it is also necessary to estimate the position in the absolute coordinate system of the center of gravity of each body element of the rigid link model and thus it further requires arithmetic processing (coordinate transformation) to represent the center of gravity location in the absolute coordinate system. Accordingly, a lot of arithmetic processing such as the coordinate transformation using the tilt angle is needed.
In this case, as a method of measuring a tilt angle relative to the vertical direction of a predetermined region of the two-legged walking mobile body, there is generally known a method of integrating a detected value of a gyro sensor for detecting an angular velocity, a method of estimating a tilt angle in the form of a so-called Kalman filter by using both of the gyro sensor and the acceleration sensor, or a method of directly detecting a tilt angle using a pendulum tiltmeter. In all of these methods, however, accumulated errors involved in an integral of detected values of the gyro sensor or an inertial acceleration during the motion of the two-legged walking mobile body often inhibits a measurement of a tilt angle relative to the vertical direction with a fully satisfactory degree of accuracy in general. Therefore, it is generally hard to grasp the tilt angle of a predetermined region of the two-legged walking mobile body in the absolute coordinate system with a high degree of accuracy. Therefore, in the method of the patent document 1 requiring a lot of arithmetic processing such as coordinate transformation using the tilt angle as described above, arithmetic errors are easily accumulated, thus inhibiting an improvement of estimation accuracy of the joint moments.
The present invention has been provided in view of the above background. Therefore, it is an object of the present invention to provide a method of estimating joint moments of a two-legged walking mobile body enabling an enhancement of estimation accuracy of the joint moments of the legs by reducing arithmetic processing using tilt information relative to the gravity direction of the two-legged walking mobile body as much as possible.